DESCRIPTION

These functions read and write extra image header fields. Writing to a field
destroys any old value. You must read a field with the correct type of reader:
you can't read an int field as a string. Fields are copied when images are
processed, so you can use them to pass information to subsequent operations.
Unless otherwise noted, image header fields created with these functions are
also saved to disc when an image is saved in VIPS format, and automatically
restored when the image is loaded again.

im_meta_set_int(3)
sets an integer field. Any existing field of any type with this name is
removed. It returns 0 for success, or -1 on error setting
im_error(3).

im_meta_get_int(3)
reads an integer field. It returns 0 for success, or -1 on error, setting
im_error(3).
It can fail if the field does not exist, or if the field is not an int.

im_meta_set_area(3)
sets a field which is an area of memory. When the field is copied to a
subsequent IMAGE, VIPS just copies the pointer. VIPS keeps a reference count
and when the last IMAGE using this field is closed, VIPS will call
free_fn
to release the memory. You can therefore use this function to attach very
large areas of memory to images efficiently. Areas cannot be saved to VIPS
files on disc, since there is no known length.